(a) Field of the Invention
This invention relates to radiography and specifically to an apparatus for orthoradial panoramic tomography.
(b) Description of the Prior Art
Orthoradial panoramic tomography also termed orthopantomography (herein called OPT-technique for short) is a known X-ray method developed and described by Y. V. Paatero since about 1960 for radiographing curved surfaces, most frequently the entire tooth-bearing area of both the upper and the lower jaw, to obtain a sharp and well defined yet flat image, photograph or other type of repreducible record, the orthoradial pantomogram also termed orthopantomogram (herein called OPT for short).
For taking an OPT of the dental or another curved portion of a human patient's head region the entire part, e.g. dentition and adjacent mandibular areas, of interest must be penetrated perpendicularly by a narrow X-ray beam passing from a radiation source to a receptor or film that is sensitive to, and indicative of, X-rays in the radiological wave length.
While pantomography can be made with a stationary apparatus and a moving object, OPT-technique uses an apparatus where both the X-ray source and the film are moved; the film, in turn, is held in a predetermined and generally curved configuration within an enclosure or cassette which, in general, is moved by secondary motion; depending upon the shape of the area or stratum of radiological interest for the particular OPT, X-ray source and cassette means will each move along a more or less complex path around the immobile head of the patient. For example, in order to take a concentric pantomogram of a cylindrical object or plane one would rotate both the X-ray source and the film cassette carrier around a single common axis of rotation which, in turn, would coincide with the axis of the cylindrical object while the cassette is also moved relative to the cassette support.
However, since typical objects of interest, such as the jaws, are not cylindrical, sharp X-ray imaging may require that the X-ray source and the cassette are moved along a particular path each involving more than one, say three or an infinite number of axes of rotation, i.e. a more or less continuously shifting position of the common axis of rotation; in other words, orthoradial pantomography of a noncylindric layer or stratum requires an excentric pantomogram including other than circular motion of the X-ray source and/or the cassette support. Numerous detailed studies about optimizing such motion for specific OPT purpose can be found in the literature, cf. Acta Orthodontica 13 (1985) 445-453.
While the particular type of movement of X-ray source and cassette or film support is not believed to be essential for the present purpose, the coordinated movement must be definitive, i.e. fully defined and reproducible and relation to an arbitrary, yet fixed, point of reference generally assumed to be positioned in a vertical plane of symmetry of the object of interest and/or the apparatus which plane, in turn, preferably will coincide with the mediosagittal plane of the patient when the OPT is taken.
For convenience and simplicity, such point is assumed herein to be located at a fixed, or fixable, point of the apparatus such that the point of reference will be within the oral cavity of the patient when an OPT is taken with the apparatus.
Another requirement for the present apparatus is that the OPT can be taken of a patient in "erect position", i.e. standing on his feet as opposed to sitting or lying; from this further requirement it will be apparent that the predetermined motion of each the radiation source and the cassette means will be in an essentially horizontal plane, i.e. intersecting perpendicularly with the standing patient's head axis in the mediosagittal plane.
OPT-apparatus meeting these requirements are available commercially from various sources; illustrative apparatus examples will be given below.
Detailed explanations of prior art OPT-apparatus will be found in the literature, e.g. U.S. Pat. No. 3,673,408 (issued June 27, 1972) and U.K. Pat. No. 2,006,590 (published May 2, 1979); such apparatus may include head positioning means comprising separate devices for the upper head portion and the lower head portion so that the patient's head can be positioned relative to the above-defined point of reference so as to obtain reasonably well-defined OPTs. However, even with the most stringent prior art head positioning devices (intraoral support or "bit" structure combined with extracranial points of contact so as to align the vertical axis of the patient's head) the OPTs, while sharp in the area of interest, are not fully reproducible, that is, "congruent" to the extent that a first and a subsequent OPT taken some time after the other could be mutually superimposed and then match in all those portions that have not changed since the first OPT was taken. In fact, the degree of reproducibility reported in the clinical studies that were discussed in the above-mentioned review by Marxer, H. in Acta Orthodontica was not sufficient to permit comparative measurements as pointed out by that author.
For diagnostic purposes, i.e. to determine time-dependent changes in a patient's head region, such as tumor diagnosis, control and treatment of undesirable dental changes and the like purposes, it would be extremely desirable to provide for congruent OPTs in the sense defined above, i.e. permitting measurements of time-dependent changes because unchanged portions would be apparent as reference positions from congruent OPTs.
The research leading to the present invention has shown that reliable congruence of sequential OPTs cannot be achieved safely with prior art apparatus even if the patient does not leave the apparatus at all and even if the time span between the first OPT and a sequential OPT is but a few minutes, not to speak of congruence of OPTs after time intervals of weeks, months or even years between the first (or preceding) and a second (or subsequent) OPT.